Had the same problem with my 2.5 v6 Rover 75 recently. Revs would often (but not always) die down to 400 rpm, then pick up to around 750. Sometimes okay, other times would just stall. I took it to the dealer I bought it from, they ran the diagnostics, no faults were logged. After some investigation, sludge and oil in the throttle body / butterfly valve was the problem causing the butterfly valve to stick. I took off the airbox rubber pipe and jubilee clip and cleaned the body out with a cloth and carb cleaner. Apparently oil gets blown back from the breather pipes connecting the throttle body to the camshaft covers. Car now works fine.